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General Genetics Chapter 14 Mendel and the Gene Idea Objectives • Understand Mendel's three Laws governing genetics • Understand the meaning of the relevant vocabulary discussed in class • Be able to predict the results of a mono & dihybrid cross using a Punnett square • Understand how recombination of genes affect genetic variability • Be familiar with special situations regarding genotype/phenotype predictions Terms from Chapter 13 • Diploid: having 2 of each chromosome type • Haploid: having a single representative of each chromosome type • Homologous chromosomes (homologs): chromosomes from different parents that are of the same type (contain similar information) • Sister Chromatids: 2 “identical” strands of DNA connected by a centromere that contains a kinetochore. Makes up each member of a homologous pair Descriptors • The physical appearance of an organism reflects its genetic makeup • Each gene codes for a different polypeptide • Polypeptide combinations may alter the appearance of an organism • Gene (Character): is a feature that is heritable • Locus: specific area on chromosome where the gene is found • Allele (Trait): is a variation of a character • Genotype: the genetic makeup of an organism (combination of genes in its nucleus) • Phenotype: the physical appearance of an organism Mendelian Genetics • Gregor Mendel described three Laws of Genetics • Law of Segregation: each parent has two copies of a gene but only one is passed to the offspring via the gametes (separation of homologous pairs) • Three possible genotypes for each gene in the diploid cell • Homozygous dominant: both alleles of a gene are of the “Dominant” variety • Homozygous recessive: both alleles of a gene are of the “Recessive” variety • Heterozygous: the diploid cell has one dominant and one recessive allele for each gene • Law of Independent Assortment: genes residing on different chromosomes separate without regard for one another – describes the broad range of variation seen in organisms • Law of Dominance: some alleles for a gene are fully expressed if present (dominant) in the phenotype while others may have their effect masked (recessive) – hierarchy of alleles Genetics is Probability • In a diploid organism (2n), each allele has a 50:50 chance of being found in a particular gamete (1/2). • To calculate the likelihood of two alleles recombining during fertilization, we must multiply our probability for each allele together (1/2 x 1/2 = 1/4) Punnett Square • Device used to predict potential genotypes of offspring • Along each axis are placed the gamete possibilities for each parent • Internal boxes represent union of genotypes for offspring produced by the union of the corresponding axial gametes • Phenotypes can be determined for each potential zygote Special Situations • Incomplete dominance: the phenotype of a heterozygous genotype is intermediate in appearance • Codominance: each allele in the genotype for a particular gene will be expressed in the phenotype • Pleiotropy: the ability of a gene to affect an organism in many ways • Epistasis: gene at one locus influences the expression of a gene at another locus (different gene) • Polygenic Inheritance: additive effect of 2 or more genes on a phenotypic character Incomplete Dominance (heterozygous genotype is intermediate in appearance) Codominance (each genotypic allele will be expressed) Pleiotropy (single gene affects multiple phenotypes) • Sickle Cell Disease – Production of abnormal hemoglobin • Prevalent amongst African Americans, but rare in people of other races Epistasis (one gene influences the expression of another gene) Polygenic Inheritance (additive effect of 2 or more genes on a phenotypic character)